Contact lenses are widely used for correcting many different types of vision deficiencies. These include low-order monochromatic aberrations such as defocus (near-sightedness or myopia and far-sightedness or hypermetropia), astigmatism, prism, and defects in near range vision usually associated with aging (presbyopia). Contact lenses must allow oxygen from the surrounding air (i.e., oxygen) to reach the cornea because the cornea does not receive oxygen from the blood supply like other tissue. If sufficient oxygen does not reach the cornea, corneal swelling occurs. Extended periods of oxygen deprivation cause the undesirable growth of blood vessels in the cornea. “Soft” contact lenses conform closely to the shape of the eye, so oxygen cannot easily circumvent the lens. Thus, soft contact lenses must allow oxygen to diffuse through the lens to reach the cornea, namely having a relatively high oxygen transmissibility (i.e., oxygen permeability over the lens thickness) from the outer surface to the inner surface to allow sufficient oxygen permeate through the lens to the cornea and to have minimal adverse effects on corneal health. High oxygen permeable silicone hydrogel materials have been developed to fulfill such requirements for making contact lenses capable of providing corneal health benefits, such as, for example, Focus NIGHT & DAY™ (CIBA VISION).
Silicone Hydrogels are typically formed of a copolymer of a polymerizable mixture including at least one hydrophilic monomer and a silicone-containing monomer or macromer. In general, silicone hydrogel contact lenses are mass-produced by a full molding process involving disposable molds. Partly because of the relatively-high cost associated with use of molds and partly because of difficulty in managing an inventory with a huge number of SKUs, a family of silicone hydrogel contact lenses made by a lens molding process generally can only have a limited number of variations in optical power and/or choices of base curve and/or the like. In most cases, a patient has to use contact lenses which would have closes match to his (her) prescription. Made-to-order or customized contact lenses can be made, e.g., by directly lathing, to match to a patient's prescription. However, it is generally believed in the art that a silicone hydrogel material can only be lathed at low temperature because of its softness and/or stickiness. Manufacturing cost could be high due to the high cost associated with low temperature lathing. Therefore, it would be desirable for a manufacturer to lathe silicone hydrogel contact lenses at room temperature.
Moreover, although silicone hydrogel contact lenses can be mass-produced economically by a full-molding process involving disposable molds, a full-molding process using disposable molds may not be suitable for making a contact lens having a relatively complex surface design because of its relatively low fidelity in reproducing all features of a lens design due to unavoidable fluctuations in the dimensions of disposable molds. Examples of contact lenses with relatively complex surface designs include without limitation contact lenses for correcting astigmatism, prism, presbyopia and customized lenses for correcting high-order monochromatic aberrations (e.g., such as a non-standard amount of spherical aberration, coma, and other irregular high-order aberrations). Those contact lenses for correcting astigmatism, prism, presbyopia and high-order monochromatic aberrations must require at least some special features built in a contact lens to provide a rotational and/or orientational stability on an eye. Their effectiveness and performance may largely depend upon both the design of orientation/stabilization features and the accurate duplication of the designs of those features. However, it is expected that, during injection-molding of molds, fluctuations in the dimensions of molds can occur as results of fluctuations in the production process (temperatures, pressures, material properties) and/or of non-uniformly shrink after the injection molding.
Therefore, there is a need for a method of lathing silicone hydrogel contact lenses at room temperature (or ambient temperature).